Metal seal for ultra high vacuum system

ABSTRACT

The present invention introduces a metal seal flange assembly for a vacuum system. A new designed metal gasket has a crosses-section shape of irregular quadrangle with two sharp angle forms by the longer base and legs. The long base of the irregular quadrangle is the vertical inner wall of the metal gasket. A preferred cross section shape of the metal gasket is trapezoid or isosceles trapezoid. This design can reduce the normal force applied to the metal seal flange assembly and reduce the number of bolts used in a limit working space.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of the U.S. application Ser. No.13/437,107, filed 2012 Apr. 2;

the entire disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a metal gasket, a vacuum flange for themetal gasket, and a vacuum seal flange assembly utilized in a vacuumseal at connections for ultra high vacuum (UHV) system.

BACKGROUND OF THE INVENTION

In assembling ultra high vacuum (UHV) systems, those systems operatingat pressures below approximately 10-6 torr, seals and closures havepresented challenging problems. For such systems, rubbers and elastomersare not suitable as seals as they are permeable to gases to an extentsuch that very low base pressures cannot be attained and such sealstypically cannot survive the high temperatures often necessary to bakeout vacuum systems. When frequent separation or proximity of heatsensitive materials makes welding impractical, plastically deformedmetal seals are satisfactory. The seals, however, require very rigidflanges and many closely spaced bolts to accomplish a sufficiently tightseal, and assembly and disassembly are time consuming. Nevertheless,plastically deformed metal seals such as gold wire and flat copperwashers are the most reliable seals and are used almost universally inUHV work. Many styles have been developed, from laboratory fabricatedspecial seals to commercially available standardized seals.

Some typical seals are illustrated in FIG. 1 a to FIG. 1 e.

-   -   (1) Crushed wire rings, of gold, copper, or aluminum—these        gaskets are generally made from round wire with desire diameter        cut to the length of the gasket mean circumference, then form        into a circle and welded. They provide positive pressures. Since        only line contact, they have high local seating stress at low        bolt loads. The contact faces increase in flowing into flange        faces. FIG. 1 a.    -   (2) Step seal with flat gasket—the soft metal gasket is crashed        and deformed in groove with two flat flange members by bolt down        force. FIG. 1 b.    -   (3) Coined gasket seal—rectangular shaped gasket is crashed in        groove by bolt down force. FIG. 1 c.    -   (4) Knife edge seal—two flanges with V-shaped ridge facing each        other with soft metal gasket in between and hold together by a        clamp or bolt. The V-shaped ridge is filled in with the material        as the gasket is deformed. FIG. 1 d.    -   (5) “Conflat” seal (Varian Association)—a soft metal gasket is        captured in a rigid structure which plastically deforms the        gasket. FIG. 1 e. Long bake out times at high temperatures        relieve internal stresses and the force on the seal, allowing        the joint to be leak-tight as differential thermal expansion is        limited.    -   (6) “Cryofit” tube fitting (Raychem Corporation)—utilizing        Nitinol alloys, a shape-memory alloy. The Cryofit connector is        essentially a sleeve of Nitinol, having internal seal ridges in        series, which is bored in the austenitic phase to a diameter        less then the outside diameter of pipes to be joined, then        chilled and transformed to martensite, and mechanically expanded        to a diameter greater than the outside diameter of pipes to be        joined. The connector can be slipped over the pipes, heated and        transformed to austenite, whereby a hoop stress presses the        series of seal ridges into the outside surface of the pipes        making an excellent pipe and hydraulic connection.    -   (7) “Helicoflex” seal (Carbone-Lorraine Industries        Corporation)—utilizing shape-memory alloy to seal the cavities,        U.S. Pat. No. 4.445.694.

All these sealing forces, external normal force bolt down or internalpressure, are applied to deform the metal gasket. The plastic deformedmetal gasket blocks the channels that connect the confined chamber andoutside environment. Thereafter the chamber can be pumped down to thedesired ultrahigh vacuum (UHV) with proper equipments.

Obara et al. in U.S. Pat. No. 4,988,130 provided a formula to estimatehow many bolts are needed to fix a pair of flanges of thickness t usingcopper gasket as seal for a pipe end of a plasma vacuum vessel. Theseflanges each have a ring-shaped knife edge. However, Obara's suggestionis for a large round-shaped pipe end with M8 bolts, which is notsuitable for a smaller size vessel with multiple opening to be sealedwith thinner M5 bolts and limit space to bolt.

The present invention provides a design of metal gasket for a limitedworking space that is easy to be plastic deformed with less number ofbolts and still provides same ability to reach desired ultrahigh vacuum(UHV) level.

SUMMARY OF THE INVENTION

The present invention provides a new design of metal gasket that provideseal for ultra high vacuum system. The metal gasket provides similar orbetter sealing effect with less tighten force and sustains long timebaking in high temperature with limited working space than aconventional gasket.

The shape of the metal gasket may be round, square, or in any shape.However, the cross section of the metal gasket is an irregularquadrangle with two sharp angles to reduce the applied bolt down force.The long base of the irregular is the vertical inner wall of the metalgasket. A preferred cross section shape of the metal gasket is trapezoidor isosceles trapezoid with two sharp angles and the short base to longbase ratio of the trapezoid is about 1:10. A preferred altitude of thetrapezoid is about one half of the long base. A preferred sharp angleformed by the long base and one leg of the trapezoid is between 30 and60 degree.

A metal seal flange assembly for a vacuum system, comprising twometallic objects having a pair of opposite surface facing each other, atleast one of the metallic objects having an opening that lead to aconfined chamber, a normal force is applied to the two opposite surfaceand form a system for vacuum; a matching groove on the second object; ametal gasket is interposed in the groove between the two metallicobjects; and the cross section of the metal gasket has a shape ofirregular quadrangle with two sharp angle form by the longer bas andlegs. The long base of the irregular is the vertical inner wall of themetal gasket. A preferred cross section shape of the metal gasket istrapezoid or isosceles trapezoid with two sharp angles and the shortbase to long base ratio about 1:10. A preferred altitude of thetrapezoid is about one half of the long base. A preferred sharp angleformed by the long base and one leg of the trapezoid is between 30 and60 degree. The normal force applied to the two opposite surface isprovided by numbers of bolts. A gas pass is cut on the second flange toexpose the dead space to vacuum.

The metal seal flange assembly has the following distinguishingfeatures: 1. The seal plane of flange is flat, the other flange has agroove to interpose the metal gasket; 2. cross section of the metalgasket is a trapezoid with two shape angle formed by the long base andthe two legs of the trapezoid; 3. the gasket deforms inward while normalforce is applied; 4. the gasket can sustain a baking temperature over150 C without gas leakage; 5. the metal seal flange assembly providessimilar or better seal outcome with smaller tightening force and lessnumber of bolts in a limited working space; 6. a gas path is introducedto expose the dead space to vacuum and thereafter reduce the outgasissue; 7. the metal seal flange assembly uses thinner metal gasket toachieve desired vacuum level; 8. the metal seal flange assembly is easyto be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIGS. 1 a-e are schematic diagrammatic representation of prior art metalseal methods, FIG. 1 a crush metal ring; FIG. 1 b step seal; FIG. 1 ccoined gasket seal; FIG. 1 d knife edge seal; FIG. 1 e Conflat flangeseal.

FIG. 2 is a photograph indicates the position of multiple holes to besealed in chamber surface.

FIG. 3 a is a schematic diagrammatic representation of a metal sealflange assembly with trapezoidal metal gasket of the present invention.

FIG. 3 b is a schematic diagrammatic representation of metal gasketdesign of the present invention.

FIG. 3 c is a schematic diagrammatic representation of a cross sectionview of the trapezoidal metal gasket according one embodiment of presentinvention.

FIG. 4 is a schematic diagrammatic representation of a cross sectionview of the flanges and metal gasket (a) before and (b) after bolt-downforce applied according one embodiment of present invention.

FIG. 5 illustrates the gas path on the seal structure that exposes thedead space to vacuum environment.

FIG. 6 is a table that compares tightening force between three differentmetal gaskets according one embodiment of present invention.

FIG. 7 is a table that compares different deformation amount of atrapezoidal metal gasket and leak test outcome according one embodimentof present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to specific embodiments of theinvention. Examples of these embodiments are illustrated in accompanyingdrawings. While the invention will be described in conjunction withthese specific embodiments, it will be understood that it is notintended to limit the invention to these embodiments. On the contrary,it is intended to cover alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims. In the following description, numerous specificdetails are set forth in order to provide a through understanding of thepresent invention. The present invention may be practiced without someor all of these specific details. In other instances, well known processoperations are not described in detail in order not to unnecessarilyobscure the present invention.

As the previous discussion, the metal gasket plastic deforms during boltdown and block the connecting channels to the environment outside theconfined chamber, thereafter produce the ultrahigh vacuum within thechamber with proper pump down equipments. A conventional metal gasket isan annular or rectangular shaped soft metal with a round or rectangularshaped cross section. Many metals are used for gasketing purposes. Someof the most common range from soft varieties such as lead, copper,steel, nickel, stainless, and Inconel to the high alloyed steels. Noblemetals such as platinum, silver, and gold are also have been used to alimit extent. In the present invention, we will use oxygen free copperas an example to explain and clarify issues we met.

The ultimate yield stress of copper is about 200 Mpa which is equivalentto 200N/mm. To plastic deform a copper gasket with a conventional roundshape cross section; the ultimate yield stress must be reached.According the formula provided in U.S. Pat. No. 4,988,130, the sealingload to deform the copper gasket requires at least 8 M8 bolts. Inpresent invention, as the FIG. 2 illustrates, the space left for sealingmultiple holes on the chamber surface is limited. It is impossible toput 8 bolts around each hole. Beside the number of bolts reduction, thepresent invention also requires thinner metal gaskets to reduce totalweight of chamber.

The purpose of present invention is to provide a metal seal for an ultrahigh vacuum system, for example a multi-axis e-beam column, with lessbolt-down force in a limited working space. Beside the limited workingspace, unlike a conventional structure, the upper (the first) flange andthe lower (the second) flange of the assembly is not symmetrical. FIG. 3a illustrates the metal seal flange assembly of the present inventionand FIG. 3 b illustrates the metal gasket for the metal seal flangeassembly. The metal gasket sit on a groove on the lower flange and theseal plane of upper flange is flat.

One embodiment of the present invention alters the shape of the crosssection of the metal gasket from conventional shape to irregularquadrangle, as the FIG. 3 c illustrates. The long base of the irregularis the vertical inner wall of the metal gasket. A preferred irregularquadrangle shape is trapezoid or isosceles trapezoid. The presentinvention will use trapezoid structure as example to illustrateembodiments of invention. The preferred trapezoid structure is, forexample, the ratio between the length of upper base to which of lowerbase of the trapezoid is about 1:10; the altitude of the trapezoid isabout half of the length of lower base; and angle form between the legand the lower base θ is between 30 and 60 degrees. This design makes thecontact plane of the upper flange 330 and the metal gasket 320 becomes aline instead of a flat surface. Reduce the contact area implies highernormal stress per unit area can be reached with a same bolt down force.Thereafter the ultimate yield stress of the metal gasket, for exampleoxygen free copper gasket, can be reach with less number of bolts andthinner bolts than what U.S. Pat. No. 4,988,130 suggested.

Metal gasket usually sits in a groove and plastic deform itself when thetwo flanges are bolted together. The plastic deformed metal gasketblocks the channels that connect the confined chamber and outsideenvironment. At the same time the deformed metal gasket produce deadspace between the two flanges and groove. During the pump down processto produce ultrahigh vacuum, the dead space will continue outgas andretard the time to reach desired vacuum level. Sometimes, the desiredvacuum level is not easy to reach if the dead space is too big. Oneembodiment of the present invention introduces a gas path 410 on thelower flange 310 to expose the dead space formed by metal gasket 320,flange 310, and flange 330 after plastic deformation as the FIG. 4 andFIG. 5 indicates. This design can eliminate the dead space outgassingissue during pump down process and less time to reach the desired vacuumlevel.

The comparison between metal gasket of trapezoidal shape cross section,hexagonal shape cross section, and conventional gasket with knife edgeflange is illustrated in FIG. 6. The gasket has trapezoid cross sectionshow no gap after tighten to the ultimate and has the smallestresistance during tighten process. This is understandable since themetal gasket of trapezoidal shape cross section has a largest normalforce when a same bolt-down force is applied.

The leak test of a metal seal flange assembly when different deformationamount is applied to the trapezoidal gasket is illustrated in FIG. 7.The leak test was performed with helium leak detection equipment. Thebest result is reached when 0.4 mm deformation which is about 20% totalreduction in height of the metal gasket was applied to the assembly. Atthis condition, the metal gasket requires medium sealing force totighten the assembly and the assembly passed the leak test before andafter 150 C, 7 hours baking test. The metal seal flange assembly with atrapezoidal metal gasket can reach a vacuum pump down to 7.6E-10 torr.

A metal seal flange assembly 300 for ultra high vacuum system compriseof two flanges and a metal gasket. Position the metal gasket 320 betweenthe flange 310 and flange 330, as the FIG. 3 a illustrates. The shape ofmetal gasket may be round, squared or in any other shape. However, thecross section of the metal gasket 320 is an irregular quadrangle. Thelong base of the irregular is the vertical inner wall of the metalgasket. A preferred shape of cross section is trapezoid, or isoscelestrapezoid. A preferred altitude of the trapezoid is about one half ofthe long base. The metal gasket 320 contacts the two flanges with thesharp angle by the longer base of the trapezoid. Provide normal forcewith bolts 340 to tight the metal seal flange assembly. Duringtightening process, the metal gasket 320 is deformed and provides sealto the metal seal flange assembly with flange 310 and flange 330. A gaspath 410 is cut on the flange 310 to expose the dead space producedbetween the metal gasket and flange 310 to vacuum environment; andthereafter resolve the outgas issue during vacuum pump down.

The metal seal flange assembly 300 has the following distinguishingfeatures: 1. The two flanges are not symmetric, one seal plane of flangeis flat, the other flange has a shallow groove to set the metal gasket;2. cross section of the metal gasket is a trapezoid with two shape angleformed by the long base and the two legs of the trapezoid; 3. the gasketdeforms inward during bolt-down; 4. the gasket can sustain a bakingtemperature over 150 C without gas leakage; 5. the seal structure canprovide similar or better seal outcome with smaller tightening force andless number of bolts in a confined working space; 6. a gas path isintroduced to expose the dead space to vacuum and thereafter reduce theoutgas issue; 7. the metal seal flange assembly uses thinner metalgasket to achieve desired vacuum level; 8. the seal structure is easy tomanufacture.

What is claimed:
 1. A metal gasket use for vacuum seal, comprising: Anannular gasket made of soft metal material with a cross section ofirregular quadrangle, wherein the irregular quadrangle has two sharpangles and the long base of the irregular quadrangle is the verticalinner wall of the annular gasket
 2. The metal gasket use for vacuum sealof claim 1, wherein the irregular quadrangle cross section shape is atrapezoid or an isosceles trapezoid.
 3. The metal gasket use for vacuumseal of claim 1, wherein the sharp angle is between 30 degree and 60degree.